Why won't my iphone update to ios 5.1?

My phone doesn't seem to have an option under "settings" to update. Is there another way to do this?

Added (1). I have an iphone 4. The current version is 4.2.8 (8E401).
My iTunes account is 10.1.1.4
I have never updated my phone before, but now some apps say I have to before I install them.
someone told me to go to "setting", "general", "Update software" - but I don't have an "update software" under settings.

Depending on which iPhone (which you didn't state) that you have it may not be able to update.

2G - 3.1.3
3G - 4.2.1
3GS, 4, 4S - 5.1.1

Those are the current latest versions of iOS that the 5 models of iPhones currently in the market can update to.

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